I replaced the pads rotors and calipers on the rear of my truck along with a new master cylinder and cannot get what seems to be the air out of the lines. The brakes will appear to work until I start the truck then nothing I can push them to the floor.

Why is my brake pedal loose once I turn the engine on? Is this air in the circuit? How do I bleed it out?

The brakes are actually bad when the engine is off. You notice it when the engine is on because the booster is assisting you. I would bet with the engine off, you could get the pedal to bottom out.
– rpmerfJun 29 '16 at 17:23

This may sound stupid, but stupid is as stupid does, and last time I changed calipers I was in such a rush I put the calipers on the wrong sides and the bleeder valve was on the bottom so when I opened the valve air would rise to the top of the caliper. So dont be in a rush and remember air rises in liquid Lol
– NateJan 26 '19 at 19:49

3 Answers
3

Some cars allow the air bubble to run out naturally, but it is rare, so you'll need an assistant.
Basically you need to apply a pressure on a brake pedal, better if you press it a few times so it becomes harder. Then you need to hold a pedal down and undo the nipple using a 7 or 8 mm spanner. Usually 1/2 of a turn is enough. The nipple is located at the top of the brake caliper (it might be located on the side, but the bore should be always at the highest point of a brake cylinder). So you let the brake fluid out, catching it and avoiding a pollution damage, better to use a clear hose pipe, put it on the nipple and other end in the bottle :) Along with a brake fluid it will push some air out, and pedal will touch the floor. Then close the nipple and AFTER THAT release the pedal. Do it again a few times and try another wheel.
REMEMBER TO CHECK AND TOP UP THE BRAKE FLUID TANK FREQUENTLY, or you'll have to do it again.
The reason why on running engine it is soft and not working is because the car is designed to make your brake pedal be softer and easier to control by using a vacuum from the engine, whether it is taken from a inlet manifold or special vacuum pump.
Your brake lines will be air-free only when you can 'lock' the wheels on a running car without 'touching a floor'. Working ABS will be counted as a locked wheels :)

If bleeding the brakes as normal isn't working you need a scantool with bi directional controls. And do an automated bleed or have the ABS module pulse the bypass modulator valve. This is required on Chevy trucks when replacing the ABS, the master cylinder or when the master cylinder goes empty.

(I have used this answer on several questions for chevy truck brake issues)

I have a 97 suburban, so I believe things are very similar. I too have replaced lines on this truck and spent hours of time and over a gallon of fluid trying to get it bled correctly. A couple things that I've learned and have helped.

Bench bleed the master. Remove the lines, install short lines, and have them go back to the reservoir on the master. Pump until no bubbles.

Bench bleed the ABS. Same as bench bleeding the master. I didn't try this, but I think it would have really helped.

When bleeding - Open the bleeder, push the pedal down, close the bleeder, release the pedal. This will help from air getting trapped.

A couple methods I've used:

Normal pump bleeding. Never quite got it right. I wasn't closing the bleeders in between pumps though.

Pressure bleeding - apply pressure to the master cylinder reservoir to push fluid through. Might had worked had the top been able to seal well, but it ended up just making a big mess.

Vacuum bleeding - apply vacuum at the bleeder. Be sure to use a ton of Teflon tape so you don't suck air from around the threads.

Reverse bleeding - Push fluid from the bleeder back through to the master.

I ended up modifying some bleeders by cutting the tip off so I could screw it all the way down to ensure no air would get past the threads. Used a ton of Teflon tape. Vacuum bleed until I purged the line, reverse bleed, install original bleeder, vacuum bleed until I don't see bubbles.

I think the reverse bleeding helps get the bubbles out near the master/abs where there is a long drop. The bubbles want to go up, and normal methods are trying to push them down. I might had had good success with bench bleeding the ABS first also.

You should also note, at least on my truck, part of the process is to have the dealer run an automated bleed on the ABS system. This takes a very specific scan tool. I tried several of my cousin's professional Snap-On scan tools, and they were not able to do the ABS bleed.